It all began when Fallon asked Emanuel to come on his show. The mayor said he would if the NBC comic came to Chicago for the plunge. Fallon called his bluff and confirmed last week that he’d join in the festivities, even dubbing himself "Swimmy Fallon."

Excited to have you @JimmyFallon! A little advice from a city that knows how to handle winter: leave the suit at home. #RahmsReaders

The duo's much-publicized Polar Plunge challenge has had more than just comedic benefits.

As of Friday morning, according to Jen Kramer, president of Special Olympics Chicago, about 3,000 had signed up for the plunge. Last year, around 2,300 people braved the elements for the annual Special Olympics fundraiser.

Kramer attributed much of the credit for this year’s spike to the famous duo and their Twitter banter.

“When Mayor Emanuel announced, we saw a little increase then,” she said. “And when Fallon committed, that’s when we saw the big increase.”

Cast members from NBC’s “Chicago Fire” and “Chicago PD” are also expected to plunge, Kramer said, along with Cook County State's Attorney Anita Alvarez and other local dignitaries. The celebrities will take their dip at 10 a.m. sharp, Kramer said. The other plungers, who collectively will have raised hundreds of thousands of dollars for Special Olympics, will head in after that.